GPS World - February 2008 - (Page 36)

SYSTEM DESIGN & TEST | Galileo p FIGURE 11 (Nav msg - IONEX) sTEC differences RMS temporal evolution from 2nd to 15th September 2007 (DOY 245-258), using a 20-degree masking angle. p FIGURE 12 Ionospheric disturbance blag for Region E Short-Term Improvements Transmission of the navigation message from GIOVE-A constitutes a major mission milestone. Some improvements have been already identiﬁed and their implementation is ongoing. The first improvements are related to reducing the latency (improving the timeliness) of the message delivery to the user by overcoming the following limitations: navigation message checks and build-up for upload are scheduled only once a day at 12:00 GMT spacecraft visibility from the GSC-A (single operational control center) can have gaps as long as 14 hours the E-OSPF is producing one single set of navigation parameters every two hours instead of producing multiple sets of navigation parameters that fit in the future prediction onboard, the navigation message parameters can only be updated on the border epochs of fixed 3-hour interval due to navigation payload constraints. The navigation message loop features that will be added are: the E-OSPF will generate, every 2 hours, multiple sets of navigation parameters for any single run. Typically, 8 x 3-hours slots messages fitting the (x,y,z) 24-hour orbit will be produced the process to uplink the navigation message from GSC-A will be run shortly before the satellite comes into visibility the start of the transmission of a new navigation message on-board will not have to respect the 3-hour boundary. Another important improvement will be the meaningful population in the navigation message of the term related to the GGTO, expected to be operational in early 2008. Currently the parameters are computed as indicated before but not yet transmitted in the navigation message. With the inclusion of these, the GIOVE-A satellite will finally become a complete navigation satellite to be used by people all over the world! Manufacturers ESA acknowledges valuable work by TAS-F, INDRA, INRIM and GMV companies on the design and development of the navigation message capability. ESA also specially thanks the two GPC operators from VEGA company, Richard Swinden and Andreas Hedqvist; without them, there would be no navigation message. MICHEL TOSSAINT is Navigation Technical Officer at the European Space Agency, the Netherlands. He graduated as a M.Sc. in aerospace engineering from Delft University of Technology, The Netherlands. correspond to the second-order polynomial that best describes the Az parameter as a function of the so-called modified dip latitude µ (MODIP) for each site. The Az coefficients estimation algorithm is in charge of the estimation of the three parameters that will be provided for single-frequency users. The validation of the Az coefficients included in the navigation message was done comparing the computed values against the IGS IONEX values on a slant level. FIGURE 11 shows the results: the maximum difference between both is 10 TECU (~1.6 m) for all the GESS locations. Ionospheric Disturbance Flag. The iono module also includes an algorithm designed to estimate the number of stations that are disturbed, that is, for which the TEC estimation is not reliable. If the number of stations that suffer from this anomaly is above a given threshold, the region containing those stations is considered disturbed, and the corresponding ionospheric disturbance flag (IDF) in the navigation message is set. FIGURE 12 shows the behavior of the flag over 50 days in 2007. The percentage of valid (1) flags was 97.88 percent, even if several of the invalid flags were caused by lack of measurements for maintenance reasons rather than for actual disturbance. 36 GPS World | February 2008 STEFANO BINDA is System Engineer in the Galileo Project at the European Space Agency. He holds a master’s degree in aeronautical engineering from the Polytechnic of Milan, Italy. JÖRG HAHN is Navigation Systems Engineer for the ESA Galileo Project Office. He received his Ph.D. in engineering sciences from the University of the Federal Armed Forces, Germany. He is a member of GPS World ’s Advisory Board. MARCO FALCONE is System Engineering Manager of the ESA Galileo Project Office. He holds a master’s in computer science from the University of Pisa, Italy, and a master’s degree in space systems engineering from the University of Delft, The Netherlands. www.gpsworld.com http://www.gpsworld.com

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GPS World - February 2008 Contents Out in Front Expert Advice Galileo's Second Coming Soon UPS First to Use ADS-B European Industry Group Galileo Validation 2008 Antenna Survey Tsumani Detection by GPS Classifieds Advertisers Index Seen + Heard

GPS World - February 2008

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